AllExceptHostEntity.php

<?php

namespace Drupal\entity_reference_test\Plugin\EntityReferenceSelection;

use Drupal\Core\Entity\Attribute\EntityReferenceSelection;
use Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Allows access to all entities except for the host entity.
 */
class AllExceptHostEntity extends DefaultSelection {
    
    /**
     * {@inheritdoc}
     */
    protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
        $query = parent::buildEntityQuery($match, $match_operator);
        
        /** @var \Drupal\Core\Entity\EntityInterface $entity */
        if ($entity = $this->configuration['entity']) {
            $target_type = $this->configuration['target_type'];
            $entity_type = $this->entityTypeManager
                ->getDefinition($target_type);
            $query->condition($entity_type->getKey('id'), $entity->id(), '<>');
        }
        return $query;
    }

}